This study will provide morphological information on the formation of sensory cell, afferent and efferent neuron synapses and on the modification of epithelial cell junctions during embryonic development of the labyrinth. Mouse, chick and amphibian embryos will be studied. Included in this study are: A. Ultrastructural examination of efferent fiber ingrowth and synapse formation, afferent calyx formation, differentiation of types I and II hair cells and freeze fracture analysis of the mature and developing intramembrane synaptic specializations in the developing chick vestibular system; B. Comparison of the morphological and temporal aspects of ribbon synapse formation in chick auditory and vestibular areas; C. Examination of the effects of VIII ganglion removal in ovo on the fine structure of avian sensory areas and D. Comparative studies of otocyst development in a mammal (mouse) and an amphibian (Ambystoma mexicanum). The systems selected provide both phylogenetic comparisons and a common basis for correlation of the varied experimental studies completed by others or planned as part of this project. Each of these systems offers potential advantages for subsequent experimentation on sensory development and seminal results from this study should suggest experimentally testable hypotheses.